The present invention relates to an improvement of an acoustic wave measuring apparatus used for measuring the dimensions of concrete materials and detecting the acoustic velocity.
Methods of measuring with the use of sound waves the thickness of a material or determining the location of flaws inside a material have been known. FIG. 1 is a schematic view of an acoustic wave measuring apparatus for measuring the thickness of a concrete material (or measuring the velocity of sound waves propagated in a concrete material of which dimensions are known) analogous to one disclosed in Japanese Patent Application Laid-open No. 63-247608 (1988). FIG. 2 is a detailed view showing more details of the apparatus of FIG. 1.
As shown in FIGS. 1 and 2, there are a concrete wall 1a to be measured in thickness, an ultrasonic generator 2 mounted directly to the surface of the concrete wall 1a, an acceleration sensor (or a wave receiver) 3 for detecting vibration of the wall, and a main unit 4 for receiving a detection signal from the wave receiver 3 and for producing and transmitting square-wave pulses to the ultrasonic generator 2. Also, shown are a square-wave pulse variable frequency oscillator 5 (referred to an oscillator hereinafter) of which oscillation frequency can be changed by a command signal, an operational amplifier 6, an NPN transistor 7, resistors 8 and 9, a buffer amplifier 10 for amplifying an output of the wave receiver 3, an A/D converter 11 for converting analog signal to digital signal, a microcomputer 12, and a memory 13. The A/D converter 11, the microcomputer 12, and the memory 13 are grouped to form a spectrum analyzer 32.
The microcomputer 12 instructs the oscillator 5 to generate square-wave pulse voltage of a predetermined frequency. The oscillator 5 transmits the square-wave pulse voltage to the ultrasonic generator 2 which in turn produces oscillation at the frequency to vibrate the target 1a to be measured. The ultrasonic generator 2, since it is driven by the square-wave pulse, emits an ultrasonic wave including a plurality of harmonics as its fundamental wave. A vibration in the target 1a in a given period of time is then measured in time sequence by the wave receiver 3, amplified by the buffer amplifier 10, with A/D converted in sequence by the A/D converter 11, saved in the memory 13, and subjected to Fourier transform for calculating the amplitude (a crest value of oscillation acceleration) of each frequency component in a range from the oscillation frequency to several times the frequency. It is noted that the buffer amplifier 10 is of a wide frequency range type which can correspond to a wide frequency range of harmonics.
Finally, the oscillation acceleration of frequencies is plotted in a graphic diagram as shown in FIG. 3, in which the horizontal axis represents the frequency and the vertical axis represents the oscillation acceleration or the crest value or amplitude. At a plurality of particular frequencies, transmission waves interfere with reflected waves to generate a standing wave. The resonance frequency of the target 1a is then expressed by the frequency with the peak of crest value, and the following equation is established. EQU 2L=n.multidot..nu./f (1)
where L is the thickness of the target or wall 1a, .nu. is the acoustic velocity across the wall 1a, f is the resonance frequency, and n is a positive integer.
When .nu. is known, L is given. When L is known, .nu. is calculated. An exemplary process for measuring the thickness is now explained assuming that .nu. is known.
It is understood that the frequency of ultrasonic waves are too high to measure sheets of concrete which have a thickness as small as several meters (i.e. n is too large in the equation (1)) because the acoustic velocity across common concrete materials is 4 to 5 km/s. Therefore, the ultrasonic generator 2 shown in FIG. 2 is replaced by a magnetostrictive vibrator which can produce lower frequencies.
FIG. 4 illustrates an arrangement of the magnetostrictive vibrator 22 in which shown are a magnetostrictive material 22a and a couple of weights 22b and 2c for increasing a force of vibration to the wall 1a. The magnetostrictive material 22a is equipped with coils and magnetized and stressed when the coils are energized, thus generating vibration when receiving an alternating current. The magnetostrictive material 22a is classified into two types, one which expands when magnetized and the other which contracts when magnetized. Both types of magnetostrictive materials expand or contract in proportion not to the direction but to the amplitude of a current applied. Therefore, it is essential for producing magnetostrictive vibration of the same frequency as that of an applied alternating current to add a direct current to the alternating current in advance.
For this purpose, the magnetostrictive vibrator 22 is provided with such a pulse voltage as shown in FIG. 5A, which is shifted to the positive side of zero and produced by the oscillator 5 as shown in FIG. 2. The frequency of the pulse voltage can be set by the microcomputer 12. The operational amplifier 6, the transistor 7, and the two resistors 8 and 9 constitute a typical voltage/current converter circuit which permits the magnetostrictive material 22 to receive a pulse current shifted to the positive side of zero.
Since the relation between current input and magnetostrictive action of the magnetostrictive material 22 is non-linear with hysteresis, the output of vibration fluctuates. The vibration output hence includes different harmonics in addition to the fundamental wave due to the generator 22 being driven by pulses. An example of the waveform of the output of the magnetostrictive vibrator 22 which is driven by a train of voltage pulses (with on-time of current 150 .mu.s and off-time 350 .mu.s at a frequency of 2 kHz as shown in FIG. 5A) is shown in FIG. 5B. For demonstrating the presence of harmonics in the output of the magnetostrictive vibrator 22, one second of the waveform for 100 kHz is Fourier-transformed and the result is spectrum-analyzed as shown in FIG. 6.
The vibration output of the magnetostrictive vibrator 22 which is driven by the continuous pulse train of voltage exhibits considerable variations in the amplitude depending on the frequencies. The frequency at which the amplitude of vibration is peaked will thus appear dislocated.
An ideal form of the vibration output of the generator for measurement includes all frequencies required for measurement and its amplitude is uniform throughout the frequencies. If some frequencies are scarce, they can hardly be detected. Variations in the amplitude cause the resonance frequency to be misread hence deteriorating the accuracy of measurement.
In addition, in the case of measuring a floor base of concrete, it is assumed that the concrete is laid on the ground or earth, whose acoustic impedance is similar to that of the concrete as compared with air or water resulting in lower reflectivity of sound wave. Also, the thicker the concrete, the less the reflected waves are received. It is not easy to read the peak of amplitude of acceleration in the vibration where the vibration increases over time (hereinafter "vibration acceleration") on a graph with the frequency plotted along the horizontal axis and the vibration acceleration expressed along the vertical axis.
The amplitude of vibration to be transmitted to a floor base is proportional to the amplitude of a measurement including reflected waves. Hence, fluctuation of the vibration output of the generator 22 directly affects the accuracy of the measurement of vibration amplitude and may disturb precise identification of the peak.
Signals of the vibration acceleration received by the wave receiver 3 have been propagated through different routes A, B . . . , as shown in FIG. 1 and are hard to examine to identify the peak frequencies in the graph shown in FIG. 3. If the resonance frequency fails to be accurately detected, measurement of the thickness will not be dependable and accordingly unsuccessful. While the waveform shown in FIG. 3 is clearly plotted for ease of description, an actual measurement waveform contains a number of noises since the buffer amplifier 10 is a wide frequency range type and easily receives noises. The crest value of each frequency component may be calculated at an acceptable level of accuracy through Fourier transformation of a succession of signals with the spectrum analyzer 32. However, for Fourier transformation of the signals sampled at small intervals of e.g. 1 Hz, the sampling of data takes one second in theory. If measurement is made at different frequencies, the sampling time for 100 frequencies will be increased up to 100 seconds.
FIG. 7 illustrates an exemplary duration of time required for detecting 100 different frequencies through sampling at intervals of 1 Hz with Fourier transformation. As shown, the measurement for 100 different frequencies takes 102 seconds. The measurement for one frequency is repeated 100 times from N1 to N100. This will increase the overall time and be low in the efficiency.
In addition to the problem of efficiency, it is common that the magnetostrictive vibrator 2 is held by an operator directly onto the concrete wall 1a. When the measurement takes substantially 100 seconds, however, the manually holding the generator 2 may vary depending on the pressure or location during the process, thus preventing the measurement from being conducted under uniform conditions.
The conventional acoustic wave measuring apparatus and method hence employ a wide frequency range type of the amplifier for processing a reflected wave thus hardly avoiding the adverse effect of noise. It is also extremely difficult to detect the resonance frequency accurately hence making the measurement difficult.
The time required for the measurement is also too long to maintain the conditions of measurement uniform.